US scientists have cloned six calves with chromosomes that show evidence of a "fountain of youth" effect.

One of the worries about previous clonings, such as Dolly the sheep, is that they are cloned from an adult cell - and that their own cells showed signs of premature ageing from birth.

But a team from a biotech company in Worcester, Massachusetts, and university colleagues report in the Science journal today that the cells of their calves show signs of lasting much longer, and dividing more often, than even those of animals conceived naturally.

The implications for agriculture, for the study of ageing and for new medical breakthroughs, such as tissue engineering and stem cell therapy, could be startling. It could mean cloned farm animals with 50% longer lifespans.

Dr Robert Lanza of Advanced Cell Technologies, said it could lead to patients with the degenerative diseases of old age could be treated with young versions of their own tissue, cultivated in laboratory dishes.

"We now know that we can generate young, healthy cells that could treat a long list of diseases caused by tissue loss or dysfunction, including heart disease, diabetes, and Parkinson's," he said. "We could seed heart cells onto a biodegradable scaffold and grow them to a substantial size. That could then be used to fix a heart like you fix the tyre of a bicycle."

All healthy cells divide a certain number of times, and then die, setting an ultimate limit to the life of the animal itself. The number of divisions seems to be linked with caps on the ends of the chromosomes called telomeres, which reduce with each division. Dolly's cells, taken from a six-year-old sheep, started prematurely old.

Using a cloning technique different from the one that produced Dolly at the Roslin Institute in Scotland in 1997, the American scientists started with calf foetal cells which had reached the end of their cell division life, were cluttered with the debris of age, and had shortened telomeres.

Out of 1,900 cell nuclei transferred to empty eggs, they managed to get six cloned calves - all born larger than normal, with high blood pressure and breathing problems, and by caesarean section. But their telomeres were significantly longer than those of normal cattle of the same age. The cells showed high levels of a gene called EPC-1, normally found only in young cells.

Long cell life, however, may not mean long life. But the cows could help answer questions about other ways that creatures age. "If the increased lifespan of the cell extends to the whole animal, these could be the longest-lived cows on the planet," said Dr Lanza. "But we don't know at this point."

The breakthrough opens the way for a whole new kind of medicine. Dr Lanza is one of the pioneers of a new kind of treatment called tissue engineering - the growing of bits of a patient's own organs outside the body for transplantation without fear of immune rejection. The extra number of cell divisions made possible by the research means that scientists could hope for a huge increase in the number of cells they could grow.

Linked with a separate approach, called embryo stem cell therapy, or therapeutic cloning, it could raise new hopes of longer, healthier lives for millions. There are 16m patients, worldwide, with neurodegenerative disorders such as Parkinson's, or Alzheimer's. More than 140m people suffer from diabetes. In the US alone, there are 64,000 patients waiting for organ transplants.

"You might be able to use this technology now, and generate healthy neurones. A disease such as Parkinson's almost immediately lends itself to this kind of therapy. You would be able to generate those cells that you were looking for in such a way that you wouldn't need to worry about immune rejection," Dr Lanza said. In the longer term, and with more research, there might be even more dramatic possibilities.

"You could even see a cure here for Aids. This technology will permit us to go back and reconstitute an entire immune system with cells that might be resistant to the Aids virus."